Local Strain Fields of LPSO in Mg-Based Ternary Alloys

Abstract

L1 2 -type TM 6 RE 8 clusters are distributed in enriched layers of LPSO (long period stacking ordered structure), and shrink in the [0001] Mg direction. The growth mechanism of LPSOs was investigated from the viewpoint of a local strain field. The strength of out-of plane normal strain of the enriched layer varied with the average radius of solute elements in the layer. The first principle calculation showed that there is a relationship between the average atomic radius of solute elements and the structural relaxation in the clusters. Zn conclusion, it is suggested that the difference of average atomic radius between Mg and solute elements controls the degree of the cluster shrinkage by the displacement of the solute elements, and that it is the dominant factor giving rise to the out-of plane normal strains of the enrichment layers.

Structural and diffusional phase transformations in liquid-quenched Mg85Y9Zn6 ribbons below the bifurcation temperature

“…The average stacking fault density close to that of the 18R is realized because the number density of clusters, or the amount of solute atoms available for the microstructure formation is equivalent to the 18R long-range order, but once a stacking fault is introduced by chance, it is persistent and takes long time to change the position even if the distance between the neighboring stacking fault is less or more than 6 atomic planes. In much dilute alloys, formation of stacking faults/ and their extension in in-plane and out-of-plane directions have been examined by Kiguchi et al [21] and Kim et al [23] by TEM/HAADF-STEM. They discussed the transition mechanism from GP zones / clusters into LPSO structures, and argued that the introduction of stacking faults and motion of Shockley partial dislocations may drive growth of LPSO.…”

“…Since the driving force of the formation of LPSO structure contains a structural origin as shown by Iikubo et al [20], and a chemical origin as shown by Masumoto et al [26], these processes may proceed concomitantly or separately depending on the heat treatment conditions. In the solute lean samples such as Mg97Y2Zn1 or Mg98.5Y1Zn0.5, clusters, or GP zones in hcp lattice tend to form on a few basal segregation layers, which later transform into LPSO block structures after introduction of stacking faults [21]- [23], rather than uniformly dispersed in hcp matrix as observed in the concentrated alloys of Mg85Y9Zn6 alloys. The schematic illustration for the transition depicted in [17] suggests that it is natural to assume that the randomly distributed clusters transformed to LPSO structure via formation of such segregated, i.e., densely populated stacking fault layers.…”

“…1. Concerning the structural transformation, Iikubo et al [20] discussed the lattice instability temperature using first principles calculations that can be athermal, while Kiguchi et al [21,22] and Kim et al [23] showed early stage of growth of the defects developing concomitant with segregation in dilute alloys. Therefore, it should be worthwhile examining the kinetics of the structural transformation process in the alloys from an isothermal transformation viewpoint.…”

Effect of Focal Depth of HAADF-STEM Imaging on the Solute Enriched Layers in Mg Alloys

“…In the current century, it has found that a novel structure, so-called "synchronized long period stacking ordered structure (LPSO)" in Mg-TM-RE (TM: transition metals, RE: rare earth metals) alloy systems that show excellent mechanical properties in comparison with the conventional Mg alloys and Al alloys such as duralmin. 13) To date, the structure and the morphology of LPSO have been clarified in atomic scale in the early studies mainly by Z-contrast imaging using high-angle annular dark fieldscanning transmission electron microscopy (HAADF-STEM). 410) 9,10) Based on the diffuse scattering pattern in the electron diffraction patterns, they concluded that L1 2 -type clusters align in inplane short range order.…”